The present invention relates to a process for producing an oxide superconductive material by firing as well as to an apparatus therefor. More particularly, the present invention relates to a process for producing an oxide superconductive material by firing, which process can suitably carry out complete firing and oxidation of a ceramic superconductive powder, etc., as well as to an apparatus therefor.
The present invention further relates to a process for producing a bottomed cylindrical oxide superconductive material by firing, in an atmosphere enabling uniform firing, a bottomed cylindrical material comprising at least two layers of (a) a bottomed cylindrical substrate layer and (b) a layer to become an oxide superconductive layer when fired, formed on the substrate layer (a).
Oxide superconductive materials have drawn people's attention in recent years because they exhibit high critical temperatures, and their application in fields such as electric power, magnetic resonance imaging, magnetic shielding and the like are expected. In magnetic shielding apparatuses, in particular, use of a bottomed (one end-closed) cylindrical superconductive material is being tried for measurement of the very weak magnetism of living body.
Cylindrical oxide superconductive materials can generally be obtained in an integral form of (a) a cylindrical substrate and (b) an oxide superconductive layer provided thereon, by forming, on a cylindrical substrate made of a metal or the like, a layer to become an oxide superconductive layer and then subjecting them to firing.
For use in production of oxide superconductive material by firing, there are known various types of firing furnaces. An electric furnace is often used for the firing of a special ceramic which can be made at a high temperature and which must be made at a precisely controlled temperature.
As the firing furnace used for the above-mentioned purpose, there is known, for example, a firing apparatus [conventional apparatus (1)] as shown in FIG. 14. In FIG. 14, a material 4 to be fired and become an oxide superconductive material is placed on a setter 7 provided in a firing furnace 2; an oxygen-rich gas is introduced into the furnace 2 through a gas inlet 1 to fire the material 4; the gas after firing is exhausted through a gas outlet 8. There is also known a firing apparatus [conventional apparatus (2)] as shown in FIG. 15. In FIG. 15, an oxygen-rich gas is introduced into a firing furnace 2 through a gas passage 9 provided outside a muffle of the furnace 2; the gas after firing is exhausted through a gas outlet 8.
In the firing of, in particular, a layer to become an oxide superconductive layer when fired, formed on a cylindrical substrate, it is impossible to use a furnace of hanging bell type, a so-called bell-shaped furnace. This bell-shaped furnace is the same as generally used for uniform heating of wire material (e.g. steel wire) or thin tube coil or for heat treatment of said linear material or thin tube in a particular atmosphere or under vacuum in a state isolated from the outside.
In the conventional apparatuses (1) and (2), however, the oxygen-rich gas introduced through the gas inlet 1 gives rise to a stream X which proceeds to the upper portion of the firing furnace 2 owing to the gas speed at the inlet 1 and a stream Y which stays at the lower portion of the firing furnace 2 owing to its temperature being lower than that inside the remainder of the furnace and then is heated by a heating element and rises to the upper portion of the furnace. When the flow rate is increased, the proportion of the stream X increases and, when the flow rate is decreased, the proportion of the stream Y increases. In either case, the stream X and the stream Y collide with each other above the material 4 to be fired, whereby turbulence in gas flow is generated, making it impossible to produce unidirectional gas flow in the vicinity of the material to be fired. When there is generated such a turbulent flow, even when the discharge gas from the gas outlet 8 contains oxygen in a high concentration of 80% or more, an oxygen-lean gas layer is present at the surface of the material 4 to be fired, as a result of an oxygen absorption reaction, as shown in FIG. 16; consequently, there remains, in the sintered material obtained, unreacted carbon derived from binder, etc. which are an organic substance or a carbonate, and the sintered material inevitably has low superconductivity.
It is an object of the present invention to provide a process for producing an oxide superconductive material by firing, in which process a gas of unidrectional flow is generated in a firing furnace so that no oxygen-lean gas layer is present on the surface of a material to be fired, placed in the firing furnace, as well as an apparatus therefor.
It is also an object of the present invention to provide a process for producing a bottomed cylindrical oxide superconductive material of desired form by firing, in which process a layer to become an oxide superconductive layer when fired, formed on a bottomed cylindrical substrate can be subjected to uniform firing.